1. Field of the Invention
This invention relates to a method for determining a rotational angle of a rotary body, and more particularly to an improved method for determining a rotational angle of a rotor of an AC motor or a DC motor which drives a door in an automatic door system in order to control the rotation of the motor.
2. Description of the Prior Art
In an automatic door system, a belt is wound around a drive pulley that is driven by an AC motor or a DC motor and a driven pulley, a door is connected to this belt, and opening and closing operations of the door is effected by normally or reversely rotating the door.
In order to control opening and closing of a door in such an automatic door system according to a desired operation mode, provision must be made such that a position and a speed (positive or negative) of the door can be always detected. In a heretofore known automatic door system, in accordance with rotation of an encoder mounted to a rotor of a motor, sensing pulses are generated at a predetermined rotational angle interval, and by count up or count down the number of these sensing pulses depending upon a direction of rotation, a moved distance of a door and hence a position of a door with reference to a given position (for instance, a closing-stop position of a door) could be known, and a moving speed of a door could be detected on the basis of a repitition cycle of these sensing pulses.
However, in such a method there is a possibility of malfunction such that in a pulse series of sensing pulses generated by rotation of an encoder upon rotation of a motor for driving a door, one pulse may drop out or one pulse is excessively added due to electrical or mechanical faults. In such cases, errors would arise in the detected position or moving speed of the door, and so it becomes impossible to precisely control opening and closing of a door in an automatic door system according to a desired operation mode.
This is not limited to an automatic door system, but the situation is also the same in operation control of other instruments driven by a motor, and more generally, this is a problem common to methods for detecting a rotational angle or a rotational speed of a rotary body by means of sensing pulses as described above.